Enhanced growth, gonad maturation, and low-level parasite infection in juvenile Manila clam Ruditapes philippinarum cultured in subtidal cages on the south coast of Korea

Abstract

The growth performance of intertidal marine bivalves is often enhanced when they are transplanted to the subtidal environment due to extended feeding time. In this study, juvenile Manila clams Ruditapes philippinarum were transplanted to subtidal suspended cages (SSC) to examine enhanced growth and reproductive activity. Juvenile clams (11.6 mm in shell length, SL) were placed in cages and suspended at 2 m depth, and shell and somatic growth, gonad maturation, and pathology were monitored. Over 103 days of the experiment, shell and somatic growth of SSC clams were significantly greater than those of the control clams cultured at the intertidal bottom (p < 0 .05). At the end of the experiment, SSC clams reached 29.0 mm in SL, whereas the intertidal bottom cage (IBC) clams only reached 18.0 mm. In late July, after 44 days in the subtidal zone, the first spawning clam was observed, and 80% of SSC clams were actively engaged in spawning by late August. In contrast, spawning in IBC was not observed until late September, and most (25%) remained sexually immature. The protozoan parasite P. olseni was detected after 30 days in the intertidal zone, increasing from 1.6 × 104 cells g−1 tissue wet weight in July to 1.30 × 106 cells g−1 tissue wet weight in September, while P. olseni infection in the SSC clams was not observed until September at much lower intensity (6.6 × 104 cells g−1 tissue wet weight). High mortality of IBC clams occurred in late July, preceded by a dramatic increase in P. olseni infection in August, suggesting that the parasitic infection was, in part, responsible for the mortalities. We speculated that prolonged feeding time in the SSC enhanced growth and gonad maturation and that the high level of parasite infection in the IBC reduced growth and reproduction further.